Porous and nonporous materials for tissue grafting and repair

ABSTRACT

Implants, such as interbody spacers, fusion devices and bone grafts, are provided having improved mechanical properties and/or degradation profiles. Such implants include a three-dimensional scaffold formed from particles, such as microspheres, which may in some embodiments be resorbable or biodegradable and which may have at least two different degradation rates. In some embodiments, the scaffold may be elastomeric. The three-dimensional scaffold may be for example, porous or semi-porous. Also provided are kits including such implants, and methods of producing and using the same.

RELATED APPLICATION

This patent application is a Divisional Application of U.S. Ser. No.11/552,244 filed on Oct. 24, 2006, now U.S. Pat. No. 8,029,575, whichclaims the benefit of priority to U.S. Provisional Patent ApplicationNo. 60/729,926 filed on Oct. 25, 2005, entitled “Method of FabricatingPorous and Non-Porous Implants for Structural Grafting Applications.”The content and subject matter of both of these applications are herebyincorporated by reference in their entireties, including all text andfigures.

FIELD

Exemplary embodiments of the present invention are generally directed todevices including three-dimensional scaffolds and/or composites havingimproved mechanical properties and/or degradation profiles, and methodsof producing the same. Exemplary embodiments also include kits andsystems including such devices in various forms, such as implantsincluding for example, synthetic bone grafts, bone graft onlays,interbody spacers, and disc repair. Exemplary embodiments furtherinclude methods of treating a patient, which include inserting into apatient in need thereof, a scaffold and/or composite formulated into adesired device.

BACKGROUND

Bone grafts used in current orthopaedic and neurosurgery proceduresoften serve two different functions. In certain surgical indications agraft is needed to act as a scaffold to aid the new bone in completelybridging a defect. In this type of graft, the material is typicallyporous to allow for bone ingrowth into the graft. Porous bone grafts actas a scaffold or trellis that allows regenerating bone to heal across adefect that it normally could not. Cancellous bone and porous ceramicshave typically been used as bone graft scaffolds. Structural grafts, onthe other hand, are bone graft materials whose main function is tomechanically support the site and add stability during the healingprocess. These materials have little to no porosity but have thestrength necessary for stabilization. Cortical bone struts, rings, andwedges are common examples of structural grafts.

The source of the bone-derived grafts is either from the patient's owncancellous or cortical bone (autograft) or from tissue donors(allograft). Although these bone grafts have been successfully used overthe years, they are not devoid of certain disadvantages. Removinghealthy bone from the patient and placing at another site often resultsin complications of pain and infection at the donor site. Using a tissuedonor can result in variable resorption characteristics andunpredictable structural integrity. In an attempt to avoid the problemsof autograft and allograft, synthetics have become a popular choice fororthopedic surgeons. Resorbable polymers, ceramics, and composites havebeen shown to be effective substitutes for bone derived grafts withoutany of the autograft or allograft related complications.

From a scaffold standpoint, current porous implants are used as graftmaterials primarily for bone and cartilage repair. These scaffolds arecharacterized by a high percent porosity to allow for bone and/orcartilage in-growth. A variety of pore forming techniques used to createthree-dimensional porous scaffolds are known. These techniques, however,result in structures that can be easily crushed or deformed due to theirlow strength. In orthopedic grafting procedures, surgeons often useforce to impact the graft material into the site, which may crush orfracture the graft material. Thus, if the graft material has lowmechanical properties, the porosity can be significantly reduced orcompletely eliminated if the graft is crushed or fractured during orafter the surgical procedure. With little to no porosity remaining, thegraft can no longer function as a scaffold for tissue in-growth.Therefore, it would be advantageous to create a tissue scaffold withhigher strength to prevent crushing during the implantation procedure.

For structural grafting applications, current implants are typicallycomposed of non-degradable materials such as titanium, poly(ether etherketone) [PEEK], and polyethylene. Although these high strength materialspossess the required mechanical properties, their function is onlyrequired during the healing process. Once the site has fully healed, theimplant serves no purpose and can be a source of long term complicationssuch as loosening or failure. This has driven the search for resorbableimplants that perform their mechanical function and are then resorbed bythe body. Polymers such as poly(lactic acid) (PLA) andpoly(lactide-co-glycolide) (PLG) have been widely used to createresorbable implants for structural applications such fracture fixation,reconstruction, and spinal fusion. However, one issue with typicalresorbable polymer implants is that they maintain their volume until thevery end of resorption. This can lead to a void in the tissue that mayor may not eventually fill in. It would be advantageous to have amaterial that would allow tissue in-growth into the implant prior tocomplete resorption of the device.

SUMMARY

Exemplary embodiments of the present invention are generally directed toimproving mechanical properties of devices, such as porous scaffolds andcomposites, which may be formed for example, into an implant suitablefor insertion into a patient. With improved mechanical properties, thedevices can resist impaction grafting without significant deformation ordestruction of the device.

Exemplary embodiments of the present invention are also generallydirected to devices, such as implants, having a step-wise degradationprofile that allows for tissue in-growth into the implant afterinsertion into a patient. This allows for improved replacement of theresorbable device by surrounding tissue.

Exemplary embodiments of the present invention are also generallydirected to creating porous scaffolds from elastomeric materials so thatthe implant can support dynamic loading as well as allow for soft tissueingrowth. With elastomeric properties and a porous structure, theimplant would be capable of supporting cyclic loading such as in thespine while allowing tissue growth into the device to form an interfacethat would prevent device migration.

Other example embodiments include methods of making such devices, kitsand systems including such devices, and methods of treating patients byinserting implants into a patient in need thereof.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention are herein described, by way ofnon-limiting example, with reference to the following accompanyingdrawings:

FIG. 1 is a schematic of a thermal consolidation process in accordancewith non-limiting example embodiments;

FIG. 2 is an illustration of the formation of a high strength compositein accordance with non-limiting example embodiments;

FIG. 3 is an illustration of effects of sintering and consolidation inaccordance with non-limiting example embodiments;

FIG. 4 is an illustration of the resorption of a two-phase particlescaffold in accordance with non-limiting example embodiments;

FIG. 5 is an illustration of a two-phase composite in accordance withnon-limiting example embodiments; and

FIG. 6 is an illustration of an implant having a particle scaffoldtherein in accordance with non-limiting example embodiments.

DETAILED DESCRIPTION

The aspects, advantages and/or other features of example embodiments ofthe invention will become apparent in view of the following detaileddescription, which discloses various non-limiting embodiments of theinvention. In describing example embodiments, specific terminology isemployed for the sake of clarity. However, the embodiments are notintended to be limited to this specific terminology. It is to beunderstood that each specific element includes all technical equivalentsthat operate in a similar manner to accomplish a similar purpose.Additionally, all of the citations herein are incorporated by referencein their entirety.

Example devices may be adapted for many different purposes and are notintended to be limited to the specific example purposes set forthherein. By way of non-limiting example, devices may include porouspolymer scaffolds that may be used for musculoskeletal repair. Othernon-limiting examples of such devices are composites that may be used,for example, for structural grafting. Those skilled in the art would beable to adapt the devices of the present invention, depending forexample, on the intended use of the device, e.g., by selecting thematerials including materials having two, three or more multipleresorption rates or degradation profiles.

Definitions

As used herein, “a” or “an” may mean one or more. As used herein,“another” may mean at least a second or more.

The terms “particle,” “particulate,” “sphere,” and “microsphere” areused interchangeably herein and are intended to encompass polymerparticles that begin somewhat discrete from one another before themethods herein are performed. These terms are also intended to includesuch particles after they are sintered, melted or otherwise bonded toone another for example to form a scaffold, although their shape maychange. These terms are also intended to include such particles,regardless of shape and even if they are compressed to become forexample, somewhat elongated in shape. Accordingly, the terms “particle,”“particulate,” “sphere,” and “microsphere” are intended to be very broadand encompass many shapes, including spheres, spheroids, rods, pellets,granules, blocks, grains or any other suitable shape.

As used herein, a “scaffold” is a device or portion thereof that allowsthe ingrowth of tissue within or upon the surface of the device. Ascaffold may be a temporary device, which may be mostly or entirelyresorbed by the body of a patient having a scaffold implanted therein.In such embodiments, tissue eventually takes up at least a portion ofphysical space occupied by the scaffold.

A “composite” is a device or portion thereof made of distinct portions.According to example embodiments, composites may include two or moreportions having different resorption profiles, that is each portionresorbs at a different rate than the other distinct portions.

A “resorbable” material may include for example ceramics, polymers orother materials that are designed to be resorbed by the body of apatient, and eventually replaced with healthy tissue. The terms“degradation rate,” “degradation profile,” “resorption rate,” and“resorption profile” are all used somewhat interchangeably herein andare intended to refer to the time over which a resorbable material isresorbed by the body. The relative terms “fast” and “slow” are usedherein to refer to the relative resorption rate of resorbable materialswith respect to each other. For example, according to some embodiments,multiple materials may be used, where one material has a so-called“fast” resorption rate and another material has a so-called “slow”resorption rate. Those skilled in the art need not know the actualresorption rates (which may vary somewhat depending on various factorssuch as material selection, placement location in the body, temperature,surrounding tissue, etc.) to understand that these embodiments areintended to include at least two discrete portions having materialcompositions that separately degrade or resorb at different rates thanone another, i.e., one composition resorbing faster than the other(s).

As used herein, “biocompatible” means the ability of an object to beaccepted by and to function in a patient without eliciting a significantforeign body response (such as, for example, an immune, inflammatory,thrombogenic, or like response).

As used herein, “polymer” means a chemical compound or mixture ofcompounds formed by polymerization and including repeating structuralunits. Polymers may be constructed in multiple forms and compositions orcombinations of compositions. The term “polymer” is also intended tocover copolymer materials and polymer blends formed from two or morepolymers. Examples of suitable polymers that may be used include, butare not limited to, biocompatible and/or bioabsorbable polymers orcopolymers and combinations thereof. Non-limiting examples include,poly(hydroxy acids), poly(phosphazenes), poly(amino acid-carbonates),poly(anhydrides), and poly(urethanes).

One of ordinary skill in the art will appreciate that the selection of asuitable polymer or copolymer depends on several factors. For example,factors in the selection of the appropriate polymer(s) that is used mayinclude bioabsorption (or bio-degradation) kinetics; in vivo mechanicalperformance; cell response to the material in terms of cell attachment,proliferation, migration and differentiation; and biocompatibility.Other relevant factors, which to some extent dictate the in vitro and invivo behavior of the polymer, include the chemical composition, spatialdistribution of the constituents, the molecular weight of the polymer,and the degree of crystallinity. The selection of polymer materialshaving varying resorption profiles would allow a multi-phase implant todegrade in a step-wise manner, thus allowing for gradual tissuein-growth. Standard techniques used in the art such as varying molecularweight, copolymer ratio, and crystallinity can be used to furtherdifferentiate degradation properties of the polymers.

As used herein, “porosity” means the ratio of the volume of intersticesof a material to a volume of a mass of the material.

The term “porous scaffold” refers to a structural matrix, which includesa solid region and an open porous region comprising spaces ordiscontinuities between adjacent areas of the solid region. The openporous region may be filled with air or gas at least initially, or itmay be at least partially filled with liquid, solid particles, gel, andthe like.

It should be understood that the macropores within the scaffold cansimply be void spaces filled with air or gas. It is also within thescope of the invention to at least partially fill some or all of thevoid spaces with a liquid, gel or solid (e.g., a plurality of particlessuch as a fine powder). The liquid, gel or solid may include one or moreactive agents. According to some embodiments, one or more active agentsmay be coated on, or otherwise incorporated into, the scaffold. By wayof non-limiting example, such “active agents” may include an antibioticsubstance, growth factors, cellular coatings, and/or other biologicallyactive substances. Thus, the biocompatible and biodegradable implant,when implanted into a cavity or extraction wound not only fills thecavity but also permits the controlled release of biologically activesubstances. For example, the substance within the pores may be selectedsuch that bacterial growth is hindered, bone formation is accelerated,and/or pain at the bone wound is reduced.

It is also within the scope of the invention to prepare an implantcomprising a shaped composite that may include few, if any, macropores(e.g., by using sufficient material to fill some or all void spaces andcreate a solid composite).

Devices Having Improved Mechanical Properties

Example embodiments include devices such as implants, which may beadvantageous for example, for structural graft applications. Suchdevices may include those comprising polymer particles bonded togetherto form all or part of a scaffold or composite. Thus, methods includeproviding a plurality of particles and bonding the particles together toform a scaffold. The particles may be bonded together for example, byheating, solvents, and/or the use of adhesives.

In particular, exemplary embodiments include a three-dimensionalscaffold made by a compression technique that includes providing aplurality of particles; compressing the particles; and bonding theparticles together to form a scaffold. Compressing the scaffold mayalter the shape of the particle, for example, making them more of anelongated shape, which may serve to increase the bonding area betweenthe particles and increase the strength. The plurality of particles mayinclude two or more particles having different shapes and/or sizes.According to example embodiments, the particles may be bonded togetherusing heat. The compression and heating may be performed at differenttimes, e.g., compressed prior to heating, or the particles may becompressed during heating. The compression of the particles may also bedone in a controlled manner to compress the particles to a specificheight. The compressed height is then essentially maintained throughoutthe heating process. The term “heating” is intended to include forexample, heating to a temperature above the polymer's glass transitiontemperature but below its melting point, by any suitable methodavailable to those skilled in the art. This compression and heatingprocess is referred to herein as a “particle compression” process.Methods implementing a “particle compression” process may include forexample, providing a plurality of particles, compressing the particles,and sintering the particles together with heat to form a scaffold.

Further, the particles can be subjected to a compressive load duringheating so that the load compresses the particles to a specific height.A heating mold can be designed to stop the compression once the desiredheight is obtained. Further non-limiting embodiments are discussed inExample 1 below.

Example methods for producing a three-dimensional scaffold includecoating a porous scaffold with a thermally stable material; and heatingthe coated scaffold past the melt temperature of the scaffold and underthe melt temperature of the thermally stable material, to thermallyconsolidate the scaffold. This coating and heating process is referredto herein as a “thermal consolidation” process. The porous scaffold maybe a porous polymer structure, e.g., a structure of particles that maybe at least partially bonded (e.g., sintered) together, or simplytouching one another, but not bonded.

Particles in any of the processes herein may include for example,particles comprising at least one resorbable polymer, biocompatiblepolymer, and/or copolymers. The particles may each comprise essentiallythe same composition, or scaffolds may include particles having two ormore compositions, such as compositions having different shapes andsizes, and/or different resorption profiles as described further herein.

Further exemplary methods for producing a three-dimensional scaffold mayinclude creating a three-dimensional, porous polymer structure; coatingthe porous polymer structure with a thermally stable material; andheating the coated structure above the melt temperature of theunderlying polymer and under the melt temperature of the thermallystable material to thermally consolidate the porous polymer structure.According to example embodiments, the heating may include repeatedheating and cooling at controlled rates to anneal the polymer. Theporous polymer structure may be for example, a plurality of particlessintered together to form a scaffold.

Methods that include thermal consolidation may further include removingthe thermally stable material after heating. Alternatively, depending onthe intended use of a resulting device, the thermally stable materialmay be left in place within the scaffold.

Examples of thermally stable material may include one or more resorbablepolymers, resorbable copolymers, resorbable polymer blends, orresorbable ceramics. The thermally stable material does notsubstantially melt or deform at the melting point of the scaffold. Thethermally stable material may include one or more biologically activeagents such as growth factors, cells, and blood factors. The coatingprocess may include filling essentially the entire porosity of thescaffold or structure with a thermally stable material, or it mayinclude coating the surface of the scaffold (e.g., coating individualparticles) while essentially retaining the scaffold's porosity.

According to example embodiments, the plurality of particles may beadded to a separate structure such as a mold or implant either before orafter forming the scaffold. According to example embodiments, theparticles may be added to the structure and are bonded (e.g., bysintering the particles together or compressing and/or heating) to forma scaffold within the structure. The structure may include for example,a mold that may be removed after formation of the scaffold, or it mayinclude an implant, such as an intervertebral spacer, interferencescrew, fracture screw, rod, suture anchor, or plate, such that thescaffold makes up only a portion of an implant to be inserted into apatient.

According to some embodiments, the particles may be sintered, compressedand/or heated within a mold. According to example embodiments theresulting scaffold may be removed from the mold or the mold removed fromthe scaffold. Particle scaffolds or structures formed in a mold may forma device such as an implant themselves, e.g., an interbody spacer, oralternatively particle scaffolds may be included, embedded, or otherwiseassociated with, a substrate to form an implant. According to someembodiments, particle scaffolds may be formed directly within a device.In cases where particle scaffolds are formed in or incorporated into adevice, the scaffold may serve to allow tissue in-growth into the devicethat previously did not allow tissue in-growth or did not allowin-growth to the same degree that the inclusion of such scaffolds willfacilitate. A non-limiting example of a particle scaffold within animplant device is described in Example 2 below. The substrate or deviceitself may be resorbable, non-resorbable, or partially resorbable.

FIG. 1 is a schematic of a thermal consolidation process in accordancewith non-limiting example embodiments. In this example, multiple polymerparticles 1 form a sintered particle scaffold 2, which is shown as thestarting three-dimensional structure. After the structure is coated witha thermally stable material 3, the coated structure 4 is heated past themelt temperature of the polymer phase. This causes the polymer phase ofthe original structure to liquefy into a highly viscous melt under thelayer of the thermally stable coating. However, the outer stable coatingessentially maintains the three-dimensional structure of the originalscaffold and prevents the collapse of the melted polymer phase. In amolten state, the polymer consolidates into a single-continuous phase 5.The consolidation of the particles into a single phase improves thestrength of the overall structure without altering the porosity. Theconsolidated particle scaffold 6 is then cooled below the melttemperature of the polymer phase and the thermally stable coating can beleft in place or subsequently removed to result in a consolidatedpolymer structure.

FIG. 2 is an illustration of the formation of a high strength compositein accordance with non-limiting example embodiments, which may be used,for example, for structural grafting. As shown in FIG. 2, a thermallystable material, such as a ceramic, in this case calcium cement, isadded to the porosity 7 of a particulate scaffold 8. The resultingcomposite 9 is then subjected to thermal consolidation, that is, thecomposite is heated to at least the melt temperature of the polymer 10,but below the melt temperature of the ceramic 11, to melt the polymerphase and fuse the particles together 12 within a consolidatedparticulate composite 13.

After consolidation is complete, the thermally stable material may bemaintained in the porosity to create a high strength composite asdepicted in FIG. 2. This composite may then be machined into a varietyof implants that would have improved properties such as increasedstrength. In addition, if the materials have different resorptionprofiles, resorption of one material before the other will give thecomposite the ability to support tissue ingrowth. This type of compositecan function, for example, as a high strength structural graft used infracture repair, spinal fusion, or other load bearing graftingapplications. If the thermally stable material is used as a coating,then the porosity may be maintained and the material can be used forexample, as tissue regeneration scaffold.

In non-limiting exemplary embodiments, a sintered PLA particulatescaffold may be filled with calcium phosphate material such as ahydroxyapatite, TCP, calcium carbonate, or calcium sulfate cement.Typical calcium cements undergo a phase transformation from liquid tosolid. In the liquid state, a calcium cement can be added to theporosity of a porous PLA scaffold using vacuum or high pressureimpregnation techniques. Once the cement hardens within the porosity ofthe particulate scaffold, the polymer-ceramic composite would be heatedto the melting point of the polymer to consolidate the particulatephase. The resulting high strength composite could then be machined intoimplants of various types such as, but not limited to, interbody fusioncages, foot and ankle fusion cages, bone graft struts, structuralaugments compatible with hip and knee arthroplasty hardware,intramedullary rods, femoral rods, fracture rods, fracture screws,fracture tacks, suture anchors, and interference screws for anteriorcruciate ligament (ACL) repair.

For polymer scaffolds, consolidation of the polymer phase can haveseveral effects on the structure depending on the technique employed. Ifa sintered particle technique is used to create the three-dimensionalscaffold, consolidation of the polymer phase through melting maysignificantly improve the particle-to-particle interface.

FIG. 3 is an illustration of effects of sintering and consolidation inaccordance with non-limiting example embodiments. As depicted in FIG. 3,loose polymer particles 14 are packed into a mold 15 and heated past theglass transition temperature of the polymer. After the scaffold isremoved from the mold 15, the sintered particles maintain theirthree-dimensional structure through weak bonding at the particle contactpoints 16. These bonds are the result of minor polymer chaininteractions between each particle. However, when the polymer phase ismelted during a thermal consolidation technique, the particlesconsolidate into a mass 17 while essentially maintaining the scaffoldshape. Thus, following consolidation, the melting of the polymer phasereplaces the weak sintered polymer chain interactions with a muchstronger thermal bond between the particles and significantly improvesthe overall strength of the polymer scaffold. Since the coating preventsthe structure from collapsing while in a molten state, the porosity ismaintained.

As indicated above, according to example embodiments, polymer scaffoldsmay be created by coating a porous structure with a thermally stablematerial that will not melt at the underlying polymer's melttemperature. As shown for example, in FIG. 1, the function of thecoating is to maintain the porous structure and prevent its collapsewhen the underlying polymer substrate is melted and consolidated.

The process of coating the thermally stable material over the porousstructure may include one or more of a variety of techniques commonlyknown to those skilled in the art. For example, Kokubo et al.,“Solutions able to reproduce in vivo surface-structure changes inbioactive glass-ceramic A-W.” J. Biomed. Mater. Res. 24:721-734, (1990),and Mooney et al., “Growth of continuous bonelike mineral within porouspoly(lactide-co-glycolide) scaffolds in vitro.” J. Biomed. Mater. Res.50:50-58 (2002), disclose methods for creating solutions of simulatedbody fluid capable of precipitating calcium phosphate ceramics of thesurface of ceramics and polymers. A variety of techniques can be used tocoat the thermally stable material to the porosity of the scaffold. Byway of non-limiting example, coating may include at least one methodselected from dip coating or solution dipping, injection molding,precipitation, vacuum impregnation, and direct application. According toexample embodiments, the coating can be as thin as 2-5 μm or cancompletely fill the porosity.

The thermally stable material may include for example a calcium-basedmaterial, such as a calcium ceramic material. For example, the thermallystable material may include one or more materials selected fromhydroxyapatite, calcium sulfate, calcium carbonate, tricalciumphosphate, biphasic calcium phosphate, and calcium sodium sulfate. Infurther example embodiments, the coating may be a 2-10 μm layer ofpoorly crystalline hydroxyapatite precipitated from a solution ofsimulated body fluid. In other example embodiments, a porous scaffoldmay be dipped coated in a low viscosity mixture of calcium sulfatehemihydrate and water (in a ratio of about 1:1, 2:1 or 3:1 calciumsulfate hemihydrate to water).

In exemplary embodiments, the coating may be a biocompatible, resorbablepolymer that has a melting point higher than the underlying polymer inthe scaffold. Once the coating is applied, the coated structure may thenbe heated to melt and consolidate the underlying polymer phase into asingle mass. The polymer is then cooled into a uniform structure. Theconsolidated structure may be then annealed through additionalcontrolled heating and cooling steps to further improve the strength ofthe polymer.

In exemplary embodiments, once the polymer substrate is consolidatedthrough the heating and cooling process, the coating may remain tofunction as an improved surface for tissue growth. With regard to bonegrafting, for example calcium coatings may remain on the surface topromote bone cell attachment.

In other example embodiments, the coating may be removed to expose theunderlying polymer scaffold and allow for thermal molding. In exampleembodiments, coatings based on calcium ceramics may be removed withexposure to mild acids such as acetic acid or 0.5 N hydrochloric acid.Polymer based coating may be removed by exposure of the coating to asolvent that will dissolve the coating only and not the underlyingscaffold. In scaffolds where the coating is removed, the consolidatedpolymer scaffold may exhibit significantly improved mechanicalproperties over scaffolds that have not been thermally consolidated.This would occur without a change in the structure and porosity of thescaffold.

According to some embodiments, the coating may be soluble in a solventthat will not substantially dissolve the polymer scaffold. In theseembodiments, the polymer coating may be applied as a solution to thescaffold such that it coats the surface of the polymer substrate withoutentirely blocking the porosity. The solvent may be evaporated and thecoating dried on the surface of the porous structure. A two-phasepolymer structure as described further herein, may then heated past themelting point of the scaffold but below the melting point of thecoating. Once consolidated, the coating may remain or be removed forexample, through dissolution in the coating's solvent.

In other exemplary embodiments, thermal consolidation can be used tocreate elastomeric structures, which may be useful for example, forgrafting applications subjected to dynamic loading. Such applicationsmay include, but are not limited to, cartilage repair, ligament repair,spinal disc repair, vascular grafting, tendon grafting, nerve grafting,spinal disc replacement, nucleus pulposus replacement, and facet jointreplacement. In these embodiments, devices can comprise, for example,resorbable or non-resorbable materials that possess elastomericproperties. For example, particles may comprise one or more resorbableor non-resorbable elastomeric materials. By way of non-limiting example,such materials may include silicone, hydrogels, polyurethane or anypolymers or copolymers exhibiting “rubbery” properties, the selection ofwhich would be within the ability of one skilled in the art.

By way of example, suitable elastomeric polymers include those thatexhibit a high percent elongation and a low modulus, while possessinggood tensile strength and good recovery characteristics. In exampleembodiments, the elastomer may exhibit a percent elongation of greaterthan about 200%. In addition to these elongation and modulus properties,suitable elastomers may also have a tensile strength greater than about4 MPa, or and a tear strength of greater than about 50 lbs/inch, orgreater than about 80 lbs/inch.

In some examples of these embodiments, the elastomeric material can beformed into particles, which are bonded together to form a scaffoldstructure. The structure may then be coated with a thermally stablematerial and heated to consolidate the elastomeric phase. The coatingwould then be removed to result in a porous scaffold that wouldessentially possess the elastomeric properties of the starting material.This type of flexible scaffold may allow for tissue in-growth, inaddition to providing dynamic load support to an implantation site.

In accordance with example embodiments, devices comprising polymerscaffolds may be adapted such that they are capable of becoming flexibleupon heating and become more rigid upon cooling. By way of example, theuse of thermoplastic polymers as the scaffold material will allow forcustom shaping at the time of surgery. Shaping can be accomplished forexample, with the addition of heat above the polymer's glass transitiontemperature. In example embodiments, use of one or more thermoplasticpolymers such as, but not limited to, poly(lactic acid) [PLA],poly(glycolic acid) [PGA], and/or poly(lactide-co-glycolide) [PLG] mayresult in thermally shapeable constructs. When exposed to temperaturesabove the glass transition point of the polymer, the scaffold becomesflexible and can be custom molded to fit the surgical site. The heatingof the scaffold can be accomplished for example, by soaking the scaffoldin hot water, or exposing the scaffold to a variety of heat sources suchas hot air gun, microwave, or a heated shaping device. As the shapedscaffold quickly cools (whether subjected to cooling or allowed to coolto the surrounding temperature), the structure maintains its newmodified shape and provides a custom fit. Devices in accordance withthese embodiments may be used for example in methods where the device isheated to become flexible, placed into a desired position, andmaintained in that position until a desired rigidity of the device isachieved.

One skilled in the art will appreciate that methods disclosed herein maybe advantageous over methods of forming polymer scaffolds usingsolvent-based techniques which produce weaker foam structures having agreater number of micropores. In solvent-based techniques a polymer isdissolved in a solvent to create a gel. The solvent is slowly removedthrough a drying process and the result is a highly porous foam that hasa broad pore size distribution. The large variety of pore sizes is dueto the extraction of the solvent during the drying steps. As a polymersolution dries, the removal of the solvent creates macropores (50-1000μm) as well as micropores (<50 μm). Although these micropores contributeto the overall percent porosity of the scaffold, they are too small fortissue in-growth and only weaken the polymer structure. In the thermalconsolidation technique disclosed herein, a thermally stable materialcan be chosen with a high enough viscosity so that essentially only thelarge pores are coated. When the coated construct is heated to themelting temperature of the polymer, the microporous polymer phase willconsolidate into a single phase with little to no microporosity. Thereduction or elimination of the unusable microporosity may significantlyimprove the overall strength of the porous composite while maintainingthe macroporosity of the scaffold.

Further encompassed by the present invention are implants comprisedentirely of or partially of at least one porous or semi-porous,elastomeric scaffold, which scaffold may be made by any suitabletechnique. Such implants may include for example, one or more of theelastomeric materials described herein and/or any suitable elastomericmaterial known to those skilled in the art. Exemplary embodiments mayinclude implants capable of supporting dynamic loading and/or implantsthat allow for soft tissue in-growth. With elastomeric properties and aporous or semi-porous structure, exemplary implants may be capable ofsupporting cyclic loading such as in the spine while allowing tissuegrowth into the implant device to form an interface that would reduce orprevent device migration. According to non-limiting exemplaryembodiments, such implants may be adapted such that they are suitablefor nucleus or disc replacement, and cartilage repair.

Also encompassed are methods of making implants comprised entirely of orpartially of porous, elastomeric scaffolds, as well as kits and systemsincluding such implants. Further encompassed are methods of treating apatient by inserting into a patient such an implant comprising at leastone porous, elastomeric scaffold.

Devices Having Multiple-Phase Degradation Capabilities

The combination of two or more materials, for example in a scaffold orcomposite, may also offer improved degradation profiles and otherproperties such as tissue in-growth. Such materials may optionallyinclude for example, two or more materials having different degradationprofiles, such as those discussed further herein. By way of non-limitingexample, devices may include three-dimensional scaffolds or compositesmade by the methods described herein, in which some particles in thescaffold comprise a composition having a first degradation profile, andother particles comprise a composition having a second degradationprofile. Other example embodiments may include any devices having two ormore materials having different degradation profiles, and may includedevices comprising scaffolds, composites or other structures made by anysuitable method.

Exemplary embodiments include a plurality of polymer particles bondedtogether to form a scaffold; where the plurality of polymer particlesinclude particles having at least two different degradation profiles.The bonding may occur by any method known to those skilled in the art,and may include for example, at least one of cross-linking, melting,sintering, compressing, and other methods of forming chemical orphysical bonds between the particles. Example embodiments includescaffolds that include a plurality of biodegradable particles having atleast two different particles shapes or sizes; wherein the particles arebound together to form a scaffold.

According to example embodiments, particle scaffolds may be fabricatedby providing a plurality of biodegradable particles, having at least twodifferent degradation rates; and bonding the particles together (forexample, by sintering) to form a scaffold. According to exampleembodiments, the particles may be subjected to the particle compressiontechnique or the thermal consolidation technique.

By way of non-limiting example, particle scaffolds may be fabricated byproviding a plurality of polymer particles comprising two or moreresorbable polymers, wherein the plurality of particles comprise atleast two polymers having at least two different degradation profiles;adding the plurality of particles to a mold or device; and sintering theparticles together to form a scaffold. These methods may further includeremoving the mold, or removing the scaffold from the mold aftersintering.

The polymer particles may include any of the polymers or combinationsthereof discussed herein, and may be selected for example, based ontheir relative rate of degradation or other characteristics depending onthe purpose for which the implant is being used. For example, suitablepolymers may include at least one polymer selected from the groupconsisting of resorbable polymers, biocompatible polymers, andcopolymers.

Exemplary embodiments of methods of making a device that includes apolymer particle scaffold may further include steps according toparticle compression or thermal consolidation techniques discussedherein. For example, such methods may include subjecting the scaffold toa thermal consolidation process comprising coating the scaffold with athermally stable material to produce a coated structure; and heating thecoated scaffold past the melt temperature of the underlying polymer tothermally consolidate the underlying polymer scaffold. The thermallystable material may be as described herein with respect to otherembodiments and may be removed or may be retained.

Accordingly, exemplary methods may include providing a plurality ofpolymer particles comprising one or more resorbable polymers; adding theplurality of particles to a structure; bonding the particles together toform a bonded particle structure having a porosity; at least partiallyfilling the porosity of the particle structure with a thermally stablematerial; and heating past the melt temperature of the particlestructure and under the melt temperature of the thermally stablematerial to thermally consolidate the particles. The structure may be amold or implant for example.

Devices made using these example methods may be adapted for varioususes. By way of non-limiting example, such devices may be adapted foruse in one or more structural grafting applications selected from thegroup consisting of spinal fusion, fracture fixation, hip and kneearthroplasty, craniofacial reconstruction, and sports medicineapplications.

In exemplary embodiments of devices that include a particle scaffold,the particles include first particles comprising a first compositioncomprising at least one polymer, wherein said first particles have afirst resorption rate; and second particles comprising a secondcomposition comprising at least one polymer, wherein said secondparticles have a second resorption rate. The first and second resorptionrates are different. The particles may also include a third or moreparticles comprising a composition having at least one polymer with yeta different resorption rate.

FIG. 4 depicts exemplary embodiments of a two-phase particle scaffoldthat can be created using particles created from two or more polymershaving different resorption profiles. As depicted in FIG. 4, thethree-dimensional structure 18 is created from a combination of a slowresorbing particles 19 and a fast resorbing particles 20. Onceimplanted, the fast resorbing particles are degraded first and result ina porosity 21, which is greater than the original porosity 22 of theoriginal three-dimensional structure 18. This increase in porosityallows for additional tissue in-growth through the structure.

An advantage over conventional scaffold techniques is that the structureof exemplary embodiments has sufficient initial mechanical strength tosurvive implantation but then has an increasing porosity as it isresorbed. Once implanted, the strength may no longer be needed andporosity increases as the fast phase is resorbed. This new greaterporosity allows for additional tissue in-growth through the scaffold. Astissue regeneration progresses, more and more of the porosity is filledwith new tissue. Unlike conventional scaffolds, as the porosity of a twoor more phase structure is filled with tissue, more porosity is createdas the quickly degrading particles are resorbed by the body. This typeof stepwise resorption profile allows the implant to possess mechanicalproperties suitable for implantation but also an increasing ability tosupport tissue in-growth as the fast resorbing phase is removed.

Further control over strength and porosity can be achieved by usingseveral different materials. This will allow the scaffold'sstrength-resorption profile to be tailored specifically for eachsurgical application.

In exemplary embodiments, various forms of the same polymer,poly(lactide-co-glycolide), may be used to give particles with varyingdegradation profiles. The copolymer 82:18 poly(L-lactide-co-glycolide)has been shown to have a degradation time of 6-9 months while 85:15poly(D,L lactide-co-glycolide) degrades in 2-3 months. In exemplaryembodiments, these polymer may be formed into microspheres and thencombined in a range of about 95:5 to 5:95 depending on the desiredresorption profile. In other example embodiments, the ratio may be about80:20 or 50:50 slow resorbing polymer to fast resorbing polymer.

In other exemplary embodiments, loose slow degrading (SD) particles andloose fast degrading (FD) particles may be mixed to create anessentially even dispersion of FD particles amongst the SD particles.The mixture may then be added to a mold and bonded (e.g., sintered) tocreate a three-dimensional particle structure. The structure could thenbe subjected to the particle compression technique or the thermalconsolidation technique described herein to further improve themechanical properties of the scaffold. In the resulting scaffold, the FDspheres would be resorbed in a period of approximately 2 to 3 months,while the SD spheres would be resorbed in approximately 6 to 9 months.Additionally, variations in the molecular weight (inherent viscosity or“IV”) of the same polymer can be used to create varying degradationprofiles. For example, fast resorbing, low molecular weight PLG polymerwith an IV range of 0.5-1.5 can be mixed with slower resorbing, highmolecular weight PLG with an IV range of 4-7.

In other example embodiments, different polymers can be used to createthe scaffold. In these embodiments, slower resorbing 70:30 poly(L/D,Llactide) particles could be mixed with faster resorbing 82:18poly(lactide-co-glycolide) particles to create a step-wise resorption.

Also included are composites comprising a plurality of portions; whereat least two portions of the plurality of portions have differentdegradation profiles. According to these embodiments, two or more phasecomposites may be formed that degrade in a manner that allows foreventual tissue in growth. In particular, a goal with creating such acomposite is that the eventual resorption of a fast degrading phase willallow space for tissue in-growth. For example, a multi-phase compositecan be created by embedding one or more faster degrading particles,e.g., fibers, rods or other shapes made of a faster resorbing materialinto a substrate made of a slower resorbing material. There may be morethan one fast degrading particle, which may have the same or differentresorption properties from one another. Additionally, a multi-phasecomposite can be created by creating a plurality of spaces independentfrom one another in a solid piece of a first material and filling saidspaces with one or more second materials that has a different resorptionprofile. A non-limiting example of a two-phase composite is described inExample 3.

The faster degrading region may span from a first side of the substrateto a second side of the substrate. The first and second sides may beadjacent to one another or opposite one another, or in the case ofrounded substrates, where there is only one side, the so-called firstand second sides are different areas of the rounded portion. Spanningfrom a first side to the second side means that either a portion of thefast degrading region is exposed on one or both or more sides, or it maymean that ends of the fast degrading region may be sufficiently close tothe sides, such that the composite may be machined to expose at leastone surface of the fast degrading particle. Embodiments where the fastresorbing region has a length sufficient to span across the composite,allows for continuous tissue in-growth from one side to the other. Inexample embodiments, the fast degrading particle may span to more thantwo sides of the substrate.

The fast degrading (FD) particle may comprise any number of a variety ofshapes, such as a fibers, rods, blocks or other shapes that may allowthe fast resorbing phase to span one side of the composite to the other.Additionally, three-dimensional fiber structures created from 3-Dbraids, weaves or meshes can be embedded in a slow resorbing substrateto create a fiber reinforced composite. Although these fibers may not bebonded together into a continuous phase, when the composite is machinedinto an implant, the individual fiber ends are exposed at both endsallowing for resorption and eventual tissue in-growth from one side ofthe composite to the other. In example embodiments, FD rods may beembedded in slow degrading (SD) substrates using injection molding ormelt processing techniques. In other example embodiments, athree-dimensional fiber braid of FD may be embedded in a SD substrateusing injection molding or melt processing.

Accordingly, example embodiments include a resorbable structuralcomposite comprising at least one fast degrading particle within aslower degrading substrate having at least a first side and a secondside; wherein the fast degrading particle has a length sufficient tospan from a first side of the composite to a second side of thecomposite. Methods for creating resorbable structural composites mayinclude for example, embedding at least one fast degrading particlewithin a slower degrading substrate, wherein the substrate has at leasta first side and a second side, to create a composite. Composites may bemachined into a device such as a plate, rod, screw (such as a fractureor interference screw), intervertebral spacer, suture anchor, or tack.

Example methods of making a resorbable structural composite may includecreating openings within a slower degrading substrate, wherein thesubstrate has at least a first side and a second side; and filling oneor more of the openings with one or more faster degrading materials tocreate a composite. The openings may be created for example by one ormore methods selected from the group consisting of drilling, milling,melting, etching, and dissolution.

According to example embodiments the slower degrading material may be abiocompatible, bioabsorbable polymer or copolymer. Example fasterdegrading material may include a calcium cement, such as those selectedfrom calcium sulfate, calcium phosphate, and calcium carbonate.

Additional example embodiments include a solid material that ismachined, drilled, or otherwise modified to create open spaces in thesolid material. Other materials with different resorption rates may thenbe added to the open spaces to fill or partially fill the space. Thespaces may completely or partially span across the first material. Thisis shown in FIG. 5.

FIG. 5 depicts a multi-phase composite in accordance with non-limitingexample embodiments. The composite may be created by machining a slowresorbing material 23 and creating open spaces 24. The spaces are thenfilled in with a fast resorbing material 25 resulting in a solidcomposite 26. The composite 26 depicted in FIG. 5 has the ability fortissue in-growth as the fast resorbing phase is degraded. In theembodiments depicted in FIG. 5, fast resorbing regions 25 are embeddedin a slow resorbing polymer 23. If portions of the fast resorbingmaterial are not already exposed, according to example embodiments, thecomposite can be machined to expose at least one surface of the fastresorbing materials and allow for tissue in-growth into the composite.When this composite is implanted in tissue, the resorption of the fastphase will allow for porosity 27 to develop within the composite. Thisnewly open porosity will allow for tissue in-growth into the composite.

It may be advantageous with respect to tissue in-growth for the fastresorbing phase to span the width of the composite as depicted in FIG.5. Although the regions are not bonded to one another in a continuousphase, they may be exposed on both sides of the composite. This allowsfor tissue in-growth from either side of the implant.

Also encompassed are implants made in whole or in part of suchcomposites. Implants may be adapted for many possible applications. Inparticular, such composites and implants made therefrom may be suitablefor surgical applications, including, but not limited to spinalinterbody fusion, fracture fixation (such as plates and screws), andstructural grafting where load resistance is required throughouthealing. Other non-limiting examples of suitable applications mayinclude suture anchors, tacks, staples, interference screws, bonefusion, bone augmentation, sports medicine, orthopaedic trauma, hip andknee replacement and sports medicine. An advantage of the multi-phasecomposite over other implant materials is that it allows for thein-growth of tissue into the composite while still providing mechanicalsupport to the site.

Kits

Example embodiments of the present invention are further directed tokits that include at least one of the scaffolds, composites or devicesthat include scaffolds, or composites of the present invention.

Numerous possible additions to kits may be contemplated by those skilledin the art reviewing this disclosure. For example, kits may furtherinclude one or more other devices, tools, materials, and the like thatmay be useful in conjunction with the scaffolds, composites or devicesof the present invention. According to example embodiments, kits mayinclude one or more devices, tools, materials and the like which mayassist in inserting the implant into a patient and/or extracting theimplant from a patient.

In embodiments where a scaffold is heated such that it becomes moldable,a kit may include a device for heating and/or cooling the scaffold at adesired time in the process of inserting the scaffold into a patient.

Kits may include tools or devices that may be useful in preparing apatient for insertion of the device into the patient.

Kits may also include one or more devices useful in placement or shapingof the device. For example, kits may include at least one component forvisualizing the device within a patient, before, during and/or afterplacement.

Kits may include ingredients for making scaffolds, composites or devicesin accordance with the present invention. For example, kits may includepolymer particles, molds, scaffolds, rods, and the like, and may furtherinclude means for bonding (e.g., by sintering), compressing orperforming any the methods encompassed by the invention.

Kits may include materials for adding biological activity to thedevices. For example, kits may include growth factors solutions orpowders, marrow aspiration needles, demineralized bone matrix powder, orother materials that may improve the healing of the device.

Systems

Exemplary embodiments of the present invention are further directed tosystems that include at least one scaffold, composite or devicesincluding such scaffolds or composites of the present invention. Suchsystems may additionally include one or more other devices, tools,materials, and the like that may be useful in conjunction with thescaffolds, composites or devices of the present invention.

Systems may include one or more devices disclosed herein and means forvisualizing the device during and/or after implantation into a patient.

Methods of Treating a Patient

Exemplary embodiments are further directed to methods of treating apatient, which include inserting into a patient in need of treatment, adevice, such as the devices described herein. The term “patient” isintended to include any animal, including, but not limited to humans.For example, methods of treatment may include veterinary applications.Those skilled in the art would be able to ascertain, which animals maybe treated by the various methods.

The reference to patients in “need of” treatment is intended toencompass situations in which treatment is medically necessary, as wellas where treatment is medically advisable, or where treatment is desiredbut optional.

The following examples illustrate non-limiting embodiments. The examplesset forth herein are meant to be illustrative and should not in any wayserve to limit the scope of the claims. As would be apparent to skilledartisans, various changes and modifications are possible and arecontemplated and may be made by persons skilled in the art.

EXAMPLE 1

In this example, the method for creating a compressed microsphereimplant is described. Poly(L/D,L lactide) is processed into microsphereswith a size range of 600-850 μm. Loose spheres are placed in a mold andcompressed to approximately 20% of their original height. The mold islocked and the approximately 20% height reduction is maintained. Thecompressed mold is then placed in an oven for sintering. Followingheating, the mold is allowed to cool and the implant is removed from themold. The compressed structure is now thermally bonded together andretains it sintered shape and the 20% compressed size. Additionally, theloose spheres can be placed in a mold outfitted with a piston and apiston stop. The stop can be set at approximately 20% of the originalheight of the loose packed spheres. During heating, the spheres softenand the weight of the piston compresses the spheres. The piston stopprevents the piston from compressing the sphere more than 20%. Once thespheres are fully compressed, the mold is cooled. Again, the compressedstructure is now thermally bonded together and retains it sintered shapeand compressed size.

EXAMPLE 2

In this example, an embodiment is depicted in FIG. 6 in which multipleresorbable polymer microsphere scaffolds 28 are formed within an implant29. The implant itself is also resorbable, and may be any number ofdifferent implants. In this particular example an intervertebral fusionimplant is depicted. The implant is composed of PLA polymer has openingsmachined into the implant wall. The microsphere scaffolds may be formedin these openings in accordance with methods of the invention. Inparticular, a PLA intervertebral implant is placed in a mold. Loosemicrospheres comprised of an approximately 50:50 mix of 82:18poly(L-lactide-co-glycolide) and 85:15 poly(D,L lactide-co-glycolide)are poured into the openings with the wall of the PLA implant. Once theopenings are filled, the mold containing the PLA implant and loosesphere is placed in the oven to sinter the spheres to one another andalso to the PLA implant walls. Once heating is complete, the implant isremoved from the mold and the implant walls are now filled with asintered multi-sphere scaffold.

EXAMPLE 3

In this example, the method for creating a two-phase structuralcomposite is described. A solid block of 70:30 poly(L/D,L lactide) ismachined to create a resorbable intervertebral implant. A variety ofholes are drilled through the solid implant to create open channels. Thechannels are then filled with a fast resorbing calcium ceramic cement(2-3 month resorption time). Once the cement hardens, the composite isfurther machined into its final implant form. The resulting structureconsists of discreet polymer and ceramic phases.

Although the invention has been described in example embodiments, manyadditional modifications and variations would be apparent to thoseskilled in the art. For example, many modifications may be made by thoseskilled in the art to the example implants. Other modifications may bemade for example to the methods, including the addition of or changingthe order of various steps. It is therefore to be understood that thisinvention may be practiced other than as specifically described. Thus,the present embodiments should be considered in all respects asillustrative and not restrictive.

1. A method for producing a three-dimensional microsphere scaffold comprising: coating a porous microsphere scaffold with a thermally stable material; and heating the coated scaffold to a temperature above melt temperature of the microsphere scaffold and under melt temperature of the thermally stable material, to thermally consolidate the scaffold.
 2. The method of claim 1, wherein the porous microsphere scaffold comprises microspheres having at least two different shapes or sizes.
 3. The method of claim 1, wherein the microspheres comprise at least one polymer.
 4. The method of claim 1, wherein the microspheres comprise at least one polymer selected from the group consisting of poly(lactic acid), poly(glycolic acid) and poly(lactide-co-glycolide).
 5. The method of claim 1, wherein the three-dimensional microsphere scaffold comprises two or more polymers having at least two different degradation profiles.
 6. The method of claim 1, wherein the porous scaffold comprises an elastomeric material.
 7. The method of claim 1, wherein the three-dimensional microsphere scaffold includes at least one biologically active agent.
 8. The method of claim 7, wherein said at least one biologically active agent is selected from the group consisting of growth factors, antibiotic substances, cells, blood factors, and demineralized bone matrix powder.
 9. The method of claim 1, wherein said scaffold is formed into a shape selected from the group consisting of a granule, a sheet, a rod, and a block.
 10. The method of claim 1, further comprising adapting said scaffold such that it may be used as at least one device selected from the group consisting of an interbody spacer, a fusion device, a bone graft, fusion cages, bone graft struts, structural augments, rods, screws, tacks, suture anchors, interference screws, a cartilage repair device, a tissue augmentation device, and a disc repair device.
 11. The method of claim 1, wherein the thermally stable material comprises at least one ingredient selected from the group consisting of resorbable polymers, resorbable copolymers, resorbable polymer blends, and resorbable ceramics.
 12. The method of claim 1, wherein said coating comprises at least partially coating the surface of the porous microsphere scaffold while essentially retaining the scaffold's porosity.
 13. The method of claim 1, wherein said coating comprises at least partially filling in the porosity of the porous microsphere scaffold with the thermally stable material.
 14. The method of claim 1, further comprising removing said thermally stable material after heating.
 15. The method of claim 1, wherein the thermally stable material remains on the microsphere scaffold after heating.
 16. The method of claim 1, further comprising adding the three-dimensional microsphere scaffold to an implant.
 17. The method of claim 16, wherein the implant is selected from the group consisting of an intervertebral spacer, rod, fracture screw, suture anchor, plate, and interference screw.
 18. A method of treating a patient comprising inserting into a patient an implant comprising the three-dimensional microsphere scaffold of claim
 1. 